This invention relates to the construction of bags of the gusseted, square-bottom type adapted to be used for the cooking of foods, such as popcorn or the like.
Gusseted square-bottom bags are disclosed in U.S. Pat. Nos. 2,596,796; 3,017,079; 3,342,402; and 4,490,131. As disclosed in these patents, the bags of this type generally comprise an outer sheet of paper and an inner liner of paper or an impervious heat sealable material such as a plastic (synthetic resin). The bag is made of a tube and the bottom end of the bag is formed by making a diamond fold comprising a triangular fold portion and a partly rectangular tab fold portion. After the triangular fold portion and the tab fold portion are sealed, the bottom of the bag is completed by folding over the tab fold portion onto a previously folded over triangular fold portion. The tab fold portion is caused to adhere to the bottom of the bag by applying suitable adhesive between the contiguous faces of the tab fold portion and the triangular fold portion of the bag bottom structure. When a bag of the indicated type is used for microwave cooking, they may be constructed in the manner described in U.S. Pat. No. 4,571,337 by the provision of a suitable coating which improves the microwave cooking operation.
In accordance with the present invention there is provided a bag bottom construction which improves the efficiency of the microwave cooking operation. To this end, and contrary to the conventional bag bottom formation as described above, the triangular fold portion is folded over to overlap the tab fold portion and the contiguous faces of these fold portions are adhered together by an adhesive which is responsive to the occurrence of an elevated temperature (such as would be produced during a microwave cooking operation) to lose at least some of its adhesive properties whereby said triangular fold portion can move away from the bag bottom to break its attachment thereto in response to a force applied thereto causing the same to move in said direction. Such a force would occur during a microwave cooking operation when the gases within the bag expand and flow from the interior of the bag past the fold at the bottom end of the triangular fold portion and into the interior thereof causing the triangular fold portion to straighten up and, therefore, move away from the bag bottom. Typically, the triangular fold portion extends directly away from the bag bottom to the farthest extent possible.